Electromagnetic radiation absorbing materials (RAMs) are useful in a range of applications. For example, they can be used as wall coatings to screen electromagnetic signals in the mobile communications or WIFI bands. They can also be used to reduce interference from wind turbines, electricity pylons, and other large metal structures, as well as being used to shield sensitive medical equipment, or sensors and wires attached to the body during magnetic resonance imaging.
At airports, surveillance radar signals reflect from airport buildings resulting in stray secondary radar emissions. To reduce these secondary emissions some airport buildings (e.g. the World Cargo Centre at Heathrow Airport, UK) are covered in radar absorbing materials.
A further use of RAMs is in the automotive industry. Current and future car designs often now include a car based radar system for aiding parking manoeuvres and monitoring car separation distances whilst en route (i.e. collision warning systems). Generally, only straight line returns from the car in front/behind are desired and so absorbent coatings may be used to cut down on spurious reflections.
Common radiation absorbing systems include Jaumann type absorbers and Salisbury screens (see U.S. Pat. No. 2,599,944), and Dallenbach type absorbers such as magnetic paint based arrangements. These types of absorbers have tended to be thick and/or heavy and have been difficult to apply or implement without adversely affecting the surface or structure to which they are applied.
Hybrid designs may take advantage of a combination of these techniques and hence, provide wider bandwidths for a reasonable thickness. However, problems in bonding, complexity in production and maintenance, and high cost are typical of such designs.
Applicant's WO 2005/013663 describes a radiation absorbing structure comprising a patterned conductor layer separated from a conducting back plane which can be less than, typically far less than, a quarter of the wavelength of absorbed radiation in thickness. This structure is particularly useful for RFID screening. However, in order to be effective for this purpose, the absorption peak should be closely aligned to the central frequency band of interest (commonly 866 MHz). This places constraints on manufacturing tolerances (physical dimensions, material permittivity values etc) or equivalently, small errors in manufacturing can lead to loss of performance.
European Patent application EP0323826 describes a multiple layered absorbing structure having a low loss, with a plurality of high loss strips running therethrough. This uses the high loss strips to absorb incident radiation of an appropriate frequency.